Why do girls have periods

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Last updated: April 8, 2026

Quick Answer: Girls have periods as part of the menstrual cycle, a biological process that prepares the body for potential pregnancy. This typically begins during puberty, between ages 10 and 15, with the average age being 12 in the United States. The cycle lasts about 28 days on average, though it can range from 21 to 35 days, and involves hormonal changes that cause the uterine lining to thicken and then shed if pregnancy doesn't occur. Menstruation usually lasts 3 to 7 days, with blood loss averaging 30-40 milliliters per cycle.

Key Facts

Menstruation typically begins between ages 10-15, with average onset at age 12 in the U.S.
The menstrual cycle averages 28 days but can range from 21 to 35 days in adults.
Menstrual flow typically lasts 3-7 days with average blood loss of 30-40 milliliters.
The menstrual cycle is regulated by hormones including estrogen, progesterone, FSH, and LH.
Menstruation continues until menopause, which typically occurs between ages 45-55.

Overview

Menstruation, commonly called a period, is a natural biological process that occurs in people with female reproductive systems as part of the menstrual cycle. Historically documented since ancient times, with early medical descriptions appearing in Egyptian papyri from 1550 BCE and Hippocratic writings from 400 BCE, menstruation has been understood through various cultural and scientific lenses. The modern understanding emerged in the 20th century with the discovery of reproductive hormones: estrogen was first isolated in 1929, progesterone in 1934, and the complete hormonal regulation of the menstrual cycle was mapped by the 1950s. Today, approximately 1.8 billion people worldwide menstruate regularly, with the average person experiencing about 450 periods in their lifetime. The menstrual cycle typically begins during puberty, which has been occurring earlier over the past century due to improved nutrition and health—in 1900, the average age of menarche was 14-15, compared to 12-13 today in developed countries.

How It Works

The menstrual cycle operates through a precisely coordinated hormonal system involving the hypothalamus, pituitary gland, ovaries, and uterus. The cycle begins with the follicular phase (days 1-14), where the hypothalamus releases gonadotropin-releasing hormone (GnRH), stimulating the pituitary to produce follicle-stimulating hormone (FSH). FSH causes ovarian follicles to mature, with one dominant follicle releasing an egg during ovulation around day 14. The follicle then transforms into the corpus luteum, producing progesterone during the luteal phase (days 15-28). Progesterone thickens the uterine lining (endometrium) to prepare for potential pregnancy. If fertilization doesn't occur, progesterone levels drop, causing the endometrium to shed as menstrual flow. This shedding involves the breakdown of blood vessels and tissue, resulting in the menstrual discharge composed of blood, endometrial tissue, and cervical mucus. The entire process is regulated by feedback loops between estrogen, progesterone, FSH, and luteinizing hormone (LH), with temperature changes and cervical mucus consistency providing observable signs of cycle progression.

Why It Matters

Menstruation has significant biological, social, and health implications. Biologically, it represents reproductive capability and overall health—irregular periods can indicate conditions like polycystic ovary syndrome (affecting 8-13% of reproductive-aged women) or thyroid disorders. Socially, menstruation affects education and economic participation, with UNESCO estimating that 1 in 10 African girls misses school during menstruation due to lack of sanitary products. The global menstrual product market exceeds $20 billion annually, yet period poverty remains an issue, with 500 million people lacking adequate menstrual hygiene management. Medically, understanding menstruation enables family planning through fertility awareness methods (75-88% effectiveness) and diagnosis of reproductive health issues. Research continues to advance, with recent studies exploring menstrual blood stem cells for therapeutic applications and improved treatments for menstrual disorders like dysmenorrhea, which affects 45-95% of menstruating people.